"The amount of salt-affected agricultural land is increasing globally due to irrigation and other factors and we need to increase the amount of food we produce every year," says Bromham.

"So we need to be able to develop crops that can deal with salt-affected land."

Bromham and colleagues were intrigued why, despite some notable successes, relatively few commercially viable, salt-tolerant cereal crops had been developed.

"Given people have been working on this for decades ... we were surprised that there hadn't been more progress," says Bromham.

This is despite the fact that there are many naturally salt-tolerant grasses in the world.

"The paradox is salt tolerance appears to arise so often in nature and yet it's so difficult to breed into crop plants," says Bromham.

Easy come and easy go

In their study Bromham and colleagues found there were at least 350 species of grasses that are naturally salt tolerant.

But when they mapped out the relationship of these species to the rest of the grass family, they were surprised at what they found.

"We expected, when we looked for those 350 species, that they'd be clustered together in family groups," says Bromham.

"[Instead] we found those species are scattered throughout the grass family."

Rather than salt-tolerant species developing a few times and then leading to a whole group of descendants with this trait, the researchers found salt tolerance evolved at least 70 separate times in the history of grasses.

Salt tolerance is a complex trait, says Bromham, but the finding that a diverse range of lineages can give rise to it, suggests it can arise quite easily.

"You don't need a big suite of traits that are very unusual to get salt tolerance," she says.

"It looks to us that normal physiology can be tweaked to become salt tolerant and that's why it can arise quite often."

But, Bromham says, the fact most salt-tolerant species are not in groups of salt-tolerant relatives suggests the trait is fleeting.

"It seems like salt tolerance can evolve fairly easily, but, on the whole, it doesn't stay around in evolutionary time," she says.

Bromham says this is possibly because salt tolerance comes at a cost to plants, and this could be the reason why it is so challenging to develop commercially viable, salt-tolerant plants.

Crop challenges

Unfortunately, the study was not able to pin down which traits resulted in the most persistent salt tolerance.

"Our results aren't going to directly lead to new crop plants but it might shed some light on why it's been so difficult to breed salt-tolerant crops," says Bromham.

An Australian expert in salt-tolerant plants, Professor Mark Tester, now at King Abdullah University of Science and Technology in Saudi Arabia, describes the new research as "very interesting".

"We have now discovered quite a few genes, but getting them into commercial lines has been slow," he says, adding there have been very few crops developed and released commercially to specifically have salinity tolerance.

"To me, [the new research] says that we have a good chance of being able to generate salt tolerant plants, but it also says to me that these adaptations come with a cost."

But Tester is confident of overcoming the challenges ahead.

"I take the positive message of Lindell's paper as encouragement, and the negative message as simply a salutary warning, not something to get disheartened about," he says.